In farming operations, chemicals such as fertilizers, herbicides and insecticides are typically applied by applicator systems mounted on implements towed by tractors over the fields. Such applicator systems generally include at least one hopper for containing a supply of the chemical, and a metering mechanism for metering the chemical into discrete streams in accordance with the desired dosage. Some applicator systems, such as the AIR-SPRED system by Gandy Company, the assignee hereof, also include pneumatic delivery techniques to effect better control during distribution of the chemicals. Such chemicals are relatively expensive, and it will be appreciated that their application is done on an acre by acre basis which is both expensive and time-consuming. Proper operation of the applicator system in turn depends upon various parameters which are preferably monitored on a continuous basis, instead of a periodic basis, so as to avoid waste of the chemical and the need to go over certain areas of the field.
One of the most basic parameters involved in operating such systems is the amount or level of chemical in the hopper. The metering mechanisms of such systems generally include adjustable openings in the bottom end of the hopper along with a feed rotor therein which function best under a certain minimum level of chemical in the hopper. If the amount of chemical drops below a certain level, imprecise metering or distribution, or complete runout of the chemical can occur. Similarly, an undue change or lack of change in the level of chemical in the hopper can signal problems with the metering mechanism or the distribution arrangement of the applicator system. The material in the hopper can be monitored on a manual basis, however, this is impractical. Continuous monitoring of the material level is desirable but is complicated by the bridging characteristics of granular materials.
Devices have been available heretofore for sensing the level of materials in hoppers, however, the devices of the prior art have not been satisfactory for a number of reasons. For example, U.S. Pat. No. 3,396,694 to Gruber and U.S. Pat. No. 2,376,304 to Anderson both relate to bin level indicators having switches or other control devices located behind flexible diaphragms mounted in openings cut into a wall of the bin. These devices are relatively complicated and bulky, and are difficult to install.
Various pressure responsive membrane switches have been developed for use in keyboards and control panels, however, these switches are adapted for localized manual push-button operation and are therefore not suited for sensing a relatively low pressure over a larger area. U.S. Pat. Nos. 4,365,408 to Ditzig; 4,349,712 to Michalski; 4,066,851 to White; and 4,065,649 to Carter are representative of these devices. Heretofore there has not been available an inexpensive membrane switch of greater sensitivity over a more generalized area which is particularly adapted for sensing granular materials in hoppers. Given the increasing emphasis placed upon application of electronics to various monitoring and control functions, together with the continuing need for more proficiency and productivity in farming operations, there is a need for such a switch.